Lithographic printing plates are discussed in U.S. patent application Ser. No. 493,528, filed Aug. 1, 1974, and entitled "Lithographic Printing Plate and Process for Making Same"*; application U.S. Ser. No. 634,899, on "Improved Lithographic Printing Plate" (Assignee's docket AP 37-B), filed on Nov. 24, 1975; and application U.S. Ser. No. 634,900, on "Improved Lithographic Printing Plate" (Assignee's docket AP 37-C), also filed on Nov. 24, 1975. The teachings of these patent applications are incorporated herein expressly by reference. FNT *Now U.S. Pat. No. 3,922,441
Paper base lithographic printing plates and methods for making the same have been well known for a considerable period of time. Lithography depends upon the immiscibility of a greasy lithographic printing ink and an aqueous etch or lithographic solution. In use, a paper lithographic printing plate is first imaged in a known manner with typed, written, or drawn copy material to be reproduced. The image may also be obtained in other ways, for instance, by xerography; e.g. Electrofax (trademark, Radio Corp. of America) and Xerox (trademark, Xerox Corp.). The grease-receptive imaging material employed makes the imaged areas ink receptive and water repellent (i.e. hydrophobic). The remaining non-imaged surface is water receptive and ink repellent (i.e. hydrophilic).
The imaged plate is placed on a plate cylinder of an offset duplicating press. The overall surface of the plate then is treated with an aqueous wet-out liquid which wets all portions of the plate except those areas that have been imaged and are water repellent. The press inking roll then passes over the surface of the plate and deposits a film of ink only upon the ink-receptive imaged areas. In the printing operation, the ink from the imaged areas is transferred in reverse to a rubber offset blanket which in turn prints directly onto a paper sheet so as to form a copy.
Desirable attributes of a lithographic or planographic printing plate include the producing of clean copy, good toning, good imaging, stop-go properties, lack of curling or wrinkling of plates while on a press, and, while accepting ink and etch, also being sufficiently water resistant so that the plate will not "milk" or "pick". The instant process produced direct image masters having the above desirable attributes, and it produces them in a rapid, efficient, and economical manner.
Conventionally, the process of making lithographic printing plates having a paper base has involved conventional sheet forming and drying of the paper on a fourdrinier paper machine and then applying to the paper base whatever coatings are desired, either in an on-machine coater or off-machine coater, typical coaters being a blade coater, an "air doctor" coater, a roll coater or the like. The fourdrinier machine itself may comprise conventionally, after the suction boxes, a press to mechanically remove water from the paper sheet followed by further water removal in an evaporative drying section, such as a series of steam heated drying cylinders. This first dryer section may then be followed by a size press, by which starch sizing is added to the paper, and a second evaporative dryer section to remove water added to the paper in the size press. The coaters then would follow the second dryer section. The primary problem with such conventional lithographic printing plate manufacture is that the paper base following drying has a relatively rough surface requiring the application of relatively thick coatings, either a barrier coat or both barrier and face coats, to achieve a satisfactory plate. The thickness of the coating requires correspondingly more intensive drying in turn limiting the speed with which the fourdrinier machine can be run, particularly if the coater is an on-machine coater.